The present invention is in the field of RF resonance spectroscopy and in particular relates to the microwave cavity structure for electron nuclear double resonance spectrometry.
Electron double resonance (ENDOR) is the phenomenon wherein nuclear resonance of sample nuclei is attained concurrently with the electron paramagnetic resonance condition for unpaired electrons of the sample material. The resonance conditions are attained in a common DC polarizing magnetic field. The sample resides within a microwave cavity, resonant at the microwave frequency for electron paramagnetic resonance (EPR) and adapted to provide the rotating RF fields requisite for nuclear magnetic resonance (NMR). Although the RF and microwave channels are in principle instrumentally independent, the ENDOR cavity imposes limitations on performance of an equivalent conventional cavity due to the presence of an RF coil. This is an extremely critical component for an ENDOR spectrometer which must sustain resonant microwave magnetic fields orthogonal to the polarizing DC magnetic field and at the same time, without degradation of the microwave cavity performance, also contain an RF coil or loop to produce the rotating RF field for the nuclear resonance.
It is an important consideration of ENDOR cavity design that the cavity Q be minimally affected by the presence of the RF loop. One prior art cavity resonant in the TE.sub.01n mode comprised a cylinder with four rods symmetrically disposed in the interior of, and at a fixed radius from the cavity axis and parallel with the cavity axis. The sample was inserted on the axis and the surrounding rods connected external to the cavity to form a pair of one-turn coils for the RF irradiation of the surrounded sample. This approach consequently required an excessively large current to produce the desired RF field intensity. The rods forming the coil, being connected external to the cavity, result in a portion of the RF energy coupled directly to the cavity closure plates through which the rods pass. This prior art cavity has been employed in equipment such as the Varian E-1700 ENDOR Spectrometer and has been described in "Multiple Electron Resonance Spectroscopy", Dorio and Freed (eds.), Plenum Press, 1979, Chptr. 2.
Another prior art ENDOR cavity operating in the TM.sub.110 mode features a cylindrical cavity with coaxial helical RF coil wound on a quartz capillary to contain the sample. Hollow metal cylinders coaxially disposed external of cavity provide mounting means for the helix. An example of this art is described in J. Chem. Phys., Vol. 61, pp. 4334-4341.